Ofcom has criticised Channel 5's attempt to give fans of Big Brother the chance to be the first to use Facebook to vote, after a server crash meant the system was unavailable during the live final.

Channel 5 used the live final on 11 November to debut technology to allow viewers to buy votes using Facebook Credits – priced at 6.5p each with a minimum purchase of between 65p and £3.26p – a first for UK television.

However, at 9.50pm during the live final, 10 minutes before the close of voting, the Big Brother Facebook app crashed leaving fans unable to cast votes and stuck with credits they had paid for which could not be used.

Channel 5 said the service was "temporarily overloaded due to exceptionally high traffic levels ... and some Facebook users were either unable to vote expediently or at all".

Ofcom was concerned that Channel 5 said it was "not liable" for repayment to viewers who had been affected as its terms and conditions stated that a technical failure was not its fault.

The media regulator said the terms and conditions do not override the broadcasting code, which "does not absolve [Channel 5] of its responsibilities to its viewers".

Channel 5 allowed the unregistered Facebook Credits votes to be refunded or used for the more recent Celebrity Big Brother series.

In addition Channel 5 said the capacity of its Big Brother Facebook application, run by "one of Europe's leading managed service providers", would be increased "seven fold" to cope with traffic peaks in the future.

Ofcom received seven complaints from viewers about the system and launched an investigation about whether Channel 5 had breached its broadcasting code, which states that the audience must not "materially mislead [viewers] about any broadcast competition or voting".

The media regulator assessed the voting information and determined that the problems with the service did not affect the overall outcome of the vote, but was nevertheless concerned at the failure.

"Channel 5 is responsible for ensuring that all of its audience voting systems are robust," the regulator said. "Ofcom was therefore concerned that the systems in place were not sufficiently robust to handle the increase in demand."

Ofcom said Channel 5 "misled" viewers as to the time they had to cast their vote "however unintentional".

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